Reference is first made to gravure printing ink. In general printing, the quality of printing depends on the printability and printing effect of printing ink. The printability is a set of properties associated with the behavior of ink from transfer of ink on a printing machine to an object to be printed to completion of a printed surface, the properties mainly including fluidity, interfacial compatibility and drying of ink. On the other hand, the printing effect is the finish of printing that the ink having formed a printed surface develops, evaluated in terms of such properties as color tone, gloss, water resistance, and blocking resistance. Ink is basically composed of a coloring matter (dye or pigment), vehicle, and auxiliary agents (flow adjusting agents, drying adjusting agents, etc.). Of these, the vehicle has the most impact on the printability and printing effect. The vehicle is basically composed of a binder, wax, solvent and the like. By a choice of the binder, the nature of the vehicle is substantially changed, on which the printability and printing effect largely depend.
The binders are generally classified into solvent binders and aqueous binders. The solvent binders include urethane resins as proposed in JP-A H09-12956 (Patent Document 1). Nowadays, however, the considerations of air pollution, fire risk and working hygiene turn attention to the aqueous binders not using organic solvents.
The aqueous binders are generally divided into alkali soluble resins as typified by styrene-acrylic resins, water-soluble resins as typified by polyvinyl alcohol, and water-dispersed resins as typified by urethane emulsions (Patent Document 2: JP-A H02-238015) and acrylic emulsions.
In general, in the case of printing using an aqueous ink comprising a water-dispersed resin as the binder, the ink is applied from a printer onto an area to be printed at room temperature, and thereafter dried at or above the glass transition temperature of the water-dispersed resin until finished. When heated at or above the glass transition temperature, overall resin particles fuse together to form a uniform film which develops and improves water resistance. With an attention paid to drying on the printer among printability factors, the printing ink must maintain fluidity on the printer for a time as long as possible without drying. In this regard, the solvent type resin, alkali-soluble resin and water-soluble resin are non-problematic in that even if the resin once becomes a dry film, the resin resumes original fluidity when dissolved in solvent, alkaline aqueous solution or water, whereas the water-dispersed resin is problematic in that once the resin is dry so that resin particles fuse together at their surface, the resin does not resume fluidity even when contacted with water, leading to a failure of printing. On the other hand, the alkali-soluble resin suffers from poor alkali resistance and the water-soluble resin suffers from poor water resistance. Then conventional binders are often prepared by combining the alkali-soluble resin, or water-soluble resin with the water-dispersed resin. In the state-of-the-art, an attempt to improve printability encounters a loss of alkali or water resistance while an attempt to improve alkali or water resistance leads to insufficient printability.
To solve the problem, JP-A H10-176132 (Patent Document 3) proposes a binder for aqueous ink comprising a monomer selected from vinyl chloride, an alkyl (meth)acrylate having an alkyl moiety of 1 to 18 carbon atoms, and a monoalkenylbenzene, an ethylenically unsaturated monomer having a functional group, and another ethylenically unsaturated monomer. However, problems remain with respect to humidity resistance and gloss.
Reference is now made to inkjet ink. The inkjet recording system is excellent in quiet, recording speed, setting quality and running cost, and is generally on widespread use. As the ink for use in this system, an aqueous ink is selected from the aspects of ink physical properties, safety, and ease of handling, ink compositions having a water-soluble dye such as an acidic dye, direct dye or basic dye, dissolved in a glycol solvent and water are known from Patent Documents 4 to 6: JP-A S53-61412, JP-A S54-89811, and JP-A S55-65269. They have the drawback of poor water resistance.
Thus the use of pigments as the coloring component is under investigation. Acrylic resins or styrene-acrylic resins having carboxyl groups incorporated therein (Patent Document 7: JP-A S61-235478) and aqueous resins in the form of ternary copolymers of acrylic acid, styrene and α-methylstyrene (Patent Document 8: JP-A H08-253716) are used to disperse pigments. Although improvements in water resistance and weather resistance are noticeable, there is left a room for improvement in dispersion (change with time) of pigments.
Next, reference is made to the receiving layer of recording media, especially of inkjet recording media. Currently the recording system using aqueous ink is frequently employed in the printer in the business machine system including personal computers, and accordingly the demand for recording material suited for that system is increasing. Meanwhile, a recording material having better characteristics is required in order to achieve further improvements in quality, colorfulness, appearance and definition of printed images. The recording material is used in diversified applications while requirements of properties such as water resistance and color development become increasingly stricter. As a typical receiving layer, there are proposed a recording sheet in which amorphous silica and a polymeric binder such as polyvinyl alcohol are combined and coated on a base paper (Patent Document 9: JP-A S55-51583) and a recording sheet comprising a base paper provided on front and back surfaces with an ink receiving layer containing porous pigment particles (Patent Document 10: JP-A S56-148584). Although a significant improvement in colorfulness or sharpness is achieved as compared with conventional recording sheets using wood-free paper, these receiving layers have the problems that a reduced coating weight allows for irregular spread of ink, resulting in printed images with substantial bleeding and that with an increased coating weight, bleeding is reduced, but the coating layer tends to spell off, causing dusting. Also, when polymeric binders such as polyvinyl alcohol are used, the viscosity of coating solution is extremely high, giving rise to the problem of poor coating viability, and the concentration of coating solution cannot be set high, giving rise to the cost problem that the water drying load is increased.
Also proposed in Patent Document 11: WO 2010/140647 is a vinyl chloride base resin emulsion obtained from emulsion-polymerization of 100 parts by weight of a monomeric composition containing a vinyl chloride monomer or a vinyl chloride monomer and an ethylenically unsaturated monomer copolymerizable therewith, in the presence of 40 to 500 parts by weight of a styrene-acrylate oligomer and/or acrylate oligomer having a number average molecular weight of 5,000 to 50,000. When this emulsion is used as aqueous ink binder or in recoding media, relatively good water resistance, color development, and alcohol resistance are exerted, but surface coating strength effect is still insufficient. Further, Patent Document 12: JP-A 2004-167784 proposes an inkjet recording sheet having a colorant-receiving layer on a support, wherein the colorant-receiving layer comprises fine particles, a water-soluble resin and a specific cationic resin, but its surface coating strength is insufficient.